Some unsaturated compounds are readily available e.g. from fossil fuel sources. However the demand for certain unsaturated compounds is greater than for others. There is therefore a need to interconvert unsaturated compounds. In particular, n-olefins may be converted e.g. by skeletal isomerization or by dimerization optionally followed by cracking. Isobutene is present in the C4 products obtained from steam-cracking of naphthas or from catalytic cracking of high boiling fuel cuts. Other C.sub.4 products such as n-butene (1-butene, cis-butene-2, trans-butene-2, and mixtures thereof) are also present. Demand for isobutene is increasing and there is therefore a need for a process for converting n-butene efficiently to isobutene.
Several processes are known for the skeletal isomerization of n-olefins e.g. the production of isobutene from n-butene. Such conversion has previously been carried out using an amorphous catalyst. For example, EP-A-152918 describes a disproportionation and isomerization catalyst for olefins which comprises silica and molybdenum oxide together with titanium dioxide as promoter.
U.S. Pat. No. 2,217,252 describes the use of natural and synthetic zeolites such as analcime, chabazite, heulandite, natrolite and thomsonite in the interconversion of olefins.
A process potentially of commercial use for the skeletal isomerization of an olefin is the so-called "Snamprogetti process". In this process the olefin is in contact with an activated alumina catalyst at high temperature (about 450.degree. C.).
The WHSV (weight hourly space velocity) is a measure of the throughput of olefin which is used in continuous use of the catalyst. At 450.degree. C. a WHSV of 2 hr.sup.-1 is common in the Snamprogetti process. Under these conditions n-butene yields approximately 30% isobutene with a selectivity of 75%. The catalyst has a life of at least 8 hours before it is necessary to regenerate it.
If the WHSV is increased to more than 12 hr.sup.-1 the isobutene yield drops to around 8%.
Another fluorine- or chlorine-containing "activated" alumina catalyst is described in EP-A-0071199. The catalytic activity is comparable with the Snamprogetti catalyst; the isobutene selectivity is higher.
U.S. Pat. No. 4,503,282 describes the use of AMS-IB boro silicate, a boron containing ZSM-5 type zeolite, in the interconversion of olefins. This catalyst operates at low WHSV (commonly 2 hr.sup.-1 or less) and high temperature (400-600.degree. C.). The reported selectivities are lower in general than those of the amorphous alumina based catalysts.
Catalysts are also used in the dimerization of n-olefins, but these are not without their problems. It is known to use as a catalyst solid phosphorous acid on kieselguhr. The catalyst cannot be regenerated and disposal of large quantities of catalyst as land-fill material is no longer acceptable .